This study seeks to demonstrate the efficacy and safety of a cost- effective and convenient treatment regimen for the delivery of aerosol tobramycin to patients with CF who are chronically colonized with P. aeruginosa, and who have evidence of moderate-severe obstructive airway disease. Specifically, this project attempts to answer the following questions: Is the administration of tobramycin (300 mg) by inhalation twice daily for 28 days efficacious and safe in patients with CF who are chronically colonized with P. aeruginosa?